Battery parts such as battery terminals, which are typically made of lead or a lead alloy, are usually cold formed in order to produce a battery terminal that is free of voids and cracks. If lead or lead alloy battery parts are pressure cast, air is left in the battery part cavity in the mold so that as the lead solidifies, the air bubbles prevent the battery part from cracking. That is, the air bubbles act as fillers so the lead remains distributed in a relatively uniform manner throughout the battery part. Unfortunately, air bubbles within the battery parts cause the battery parts to be rejects as the air bubbles can produce large voids in the battery part. In order to minimize the air bubbles in the battery part, a vacuum can be drawn in the battery part cavity mold; however, although the vacuum removes air from the mold and inhibits the forming of air bubbles in the battery part, the battery parts cast with a vacuum in the battery part cavity oftentimes solidify in an uneven manner producing battery parts with cracks or tears which make the battery parts unacceptable for use.
The process of pressure casting or die casting of battery parts wherein intensification of the battery part is used to form battery parts substantially free of cracks and tears is more fully described in my co-pending applications U.S. Ser. No. 09/170,247, filed Oct. 13, 1998, titled Apparatus for and Method of Casting Battery parts; U.S. Ser. No. 09/458,198 filed Dec. 10, 1999, now U.S. Pat. No. 6,564,853, titled Multiple Casting Apparatus and Method, and U.S. Ser. No. 09/321,776 filed May 27, 1999, now U.S. Pat. No. 6,405,786, titled Apparatus and Method of Forming Battery Parts which are herein incorporated by reference.
In one such embodiment, a battery part is cast which is substantially free of cracks and tears by pressure casting a lead alloy while a vacuum is being applied to the battery part cavity. At the moment when the lead in the battery part cavity reaches the liquid-to-solid transformation stage, the part is intensified by driving a piston into the mold cavity to rapidly reduce the volume of the mold for solidification. By precisely controlling the time of application of an external compression force to the molten lead in the battery part cavity, and consequently, the time at which the volume of the battery part cavity is reduced, one can force the molten lead or lead alloy in the flowable state into a smaller volume where the pressure on the battery part cavity is maintained. By maintaining the pressure on the battery part cavity during the solidification process by intensification (driving a piston into the lead), the battery part can be cast in a form that is substantially free of cracks and tears.
In another embodiment, the mold for forming the pressure cast battery part is sealed off while the molten lead is still in the molten state and before the molten lead can begin to solidify the supply of pressurized lead is shut off and at the same time the internal pressure of the molten lead is increased by driving a piston into the molten metal. This intensification process is suited for those applications where the entire mold can withstand the higher pressures. That is, when the liquid metal is in a molten state an increase in pressure of the molten lead throughout the mold and the maintaining of the increased pressure during solidification can produce a battery part free of tears and cracks. This process of intensification by driving a piston into the mold allows one to obtain greater molding pressure than is available with conventional pressure casting techniques.
In another embodiment, the cast battery part is subjected to at least a partial cold forming during the volume contraction step by rapidly driving a piston into the solidified cast battery part with sufficient force to cold form a portion of the lead in the battery part to thereby produce a battery part that is free of cracks and tears. This method of partial cold form intensification is more suitable for those battery parts where one does not want to subject the mold to excessively higher pressures than the die casting pressures.
In the present invention, a finished battery, which is die cast and substantially free of cracks and tears, is formed by extending a piston that first shutoffs the flow of molten lead into and out of the mold cavity. Further extension of the piston brings a piston face that forms a bounded end face of the mold part toward the other faces of the mold. Instead of driving a piston into the mold cavity to increase the pressure of the die cast battery part the entire mold face is brought toward the set of other mold faces to decrease the volume of the mold. Thus the shutoff and intensification are accomplished by a single stroke of an extendible piston carrying a mold face thereon.
By finished surface it is meant that the surface of the battery does not contain flashing or irregularities where the molten lead was supplied through a gate. That is, in die casting the runner that supplies lead to the mold is usually broken off when the battery part is removed from the mold thus leaving an unfinished surface. Since irregularities can create problems in electrical operation of the battery part it is desired to have a smooth finished surface over the entire battery part. As pointed out, such finished surfaces are usually obtained only with cold forming a battery part. The present invention provides such a finished surface without having to cold form the battery part. In addition, one can also increase the pressure sufficiently to inhibit voids and cracks in the battery part.
In the present invention a retractable piston has an impact surface or mold face that forms an entire side-to-side mold face or mold surface of the battery part thereby eliminating the formation of a local irregularity in the portion of the surface of the battery part that would occur if the piston penetrated a portion a mold face i.e. breaking the surface plane of the mold cavity. The use of a side-to-side or bounded mold face that does not break the surface plane of the mold cavity substantially eliminates the need to finish the battery part. That is, once the part is removed from the mold it is ready for use since the surface plane of the mold cavity has not been broken or penetrated by the moving end face. In addition, since the entire side-to-side surface of the battery part is impacted the precision timing of the intensification step is eliminated. That is, since the intensification pressure is applied on a side-to-side portion of the battery part cavity the lead can be in either the liquid, solid or mush state since the all the lead can be confined and squeezed within the cavity of the battery part mold cavity.